Many portable electronic devices such as cameras, cellular telephones, personal digital assistants (PDAs), MP3 players, computers and other devices include an imager for capturing images. One example of an imager is a complementary metal-oxide semiconductor (“CMOS”) imager. A CMOS imager includes a focal plane array of pixels, each one of the pixels including a photosensor, for example, a photogate, photoconductor or a photodiode overlying a substrate for accumulating photo-generated charge in the underlying portion of the substrate. Each pixel has a readout circuit that includes at least an output field effect transistor and a charge storage region connected to the gate of the output transistor. The charge storage region may be constructed as a floating diffusion region. Each pixel may include at least one electronic device such as a transistor for transferring charge from the photosensor to the storage region and one device, also typically a transistor, for resetting the storage region to a predetermined charge level prior to charge transference, and a row control transistor for selectively connecting the pixel to a column line.
In a CMOS imager, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) resetting the storage region to a known state; (4) transfer of charge to the storage region accompanied by charge amplification; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing a reset level and pixel charge. Photo charge may be amplified when the charge moves from the initial charge accumulation region to the storage region. The charge at the storage region is typically converted to a pixel output voltage by a source follower output transistor.
FIG. 1 illustrates a typical four-transistor pixel 50 utilized in a pixel array of an imager, such as a CMOS imager. The pixel 50 includes a photosensor 52 (e.g., photodiode, photogate, etc.), transfer transistor 54, and readout circuit 51. The readout circuit 51 includes a storage node configured as a floating diffusion region N, reset transistor 56, source follower transistor 58 and row select transistor 60. The photosensor 52 is connected to the floating diffusion region N by the transfer transistor 54 when the transfer transistor 54 is activated by transfer control line 53 carrying a transfer control signal TX. The reset transistor 56 is connected between the floating diffusion region N and an array pixel supply voltage Vaapix. A reset control signal RST supplied over a reset control line 57 is used to activate the reset transistor 56, which resets the floating diffusion region N to a known state as is known in the art.
The source follower transistor 58 has its gate connected to the floating diffusion region N and is connected between the array pixel supply voltage Vaapix and the row select transistor 60. The source follower transistor 58 converts the charge stored at the floating diffusion region N into an electrical output signal. The row select transistor 60 is controllable by a row control signal ROW supplied over a row control line 61 for selectively outputting the output signal OUT from the source follower transistor 58 to a sample and hold circuit 46 via column line 45. For each pixel 50, two output signals are conventionally generated, one being a reset signal Vrst generated after the floating diffusion region N is reset, the other being an image or photo signal Vsig generated after charges are transferred from the photosensor 52 to the floating diffusion region N. Output signals Vrst Vsig are selectively stored in the sample and hold circuit 46 based on reset and pixel signal sample and hold control signals SHR, SHS.
Pixel 50 is included in one or more pixel arrays for use in an imager. The pixels in a pixel array are arranged in rows and columns, such that each pixel detects the light intensity at the location of that pixel. Each row in the pixel array may include a row driver configured to send a plurality of signals to an identified pixel or row of pixels. The signals generated by the row drivers include row control signals ROW, reset control signals RST and transfer control signals TX, among others. Thus, a row driver includes circuitry for generating, for example, a reset control signal RST or a transfer control signal TX. Such circuitry within the row driver is referred to as a reset row driver or a transfer row driver.
Reset row drivers and transfer row drivers may include boost circuitry that allows for the amplification or boosting of the reset and transfer control signals RST, TX. By boosting the voltage of the reset and transfer control signals at appropriate times, the dynamic range of the imager pixels can be increased without increasing a supply voltage VAA to the imager. Dynamic range refers to the range of incident light that can be sensed by an image sensor in a single frame of pixel data. It is desirable to provide boosted control signals only when necessary, however, in order to reduce the load on the boost circuitry.
Imager row drivers provide the generated control signals to specific pixels at specific times, as determined by a timing and control circuit of the imager. Some imagers utilize a rolling shutter mechanism which affects the timing of the row driver control signals. In a rolling shutter device, a pixel array is incrementally exposed to light from an image, row-by-row. For example, the top or first row in the pixel array may be exposed first to light from an image. Soon thereafter, the second row is exposed, followed by the third row, and so forth. Each pixel row has a same total integration time, but each row begins its integration time (and hence also ends its integration time) at different times. Similarly, the control signals for each row are each issued at different times.
Additional image-obtaining methods may also be used in imagers. For example, various scaling modes may be employed. When every pixel from every row of the pixel array is sampled and used in an image, pixel-by-pixel, the imager is utilizing a full resolution mode. Though full resolution mode results in the highest possible level of detail, other modes may sometimes be used in order to reduce processing times, memory requirements and noise. Skipping mode results in the utilizing of pixels from only some selected rows of the pixel array while other rows are skipped. For example, a pixel array operating in skipping mode may only read out the pixel values of every other row of pixels. In windowing mode, only pixels from a specified block or window of pixels are readout. In binning mode, selected pixel outputs from adjacent rows are summed together and selected outputs from adjacent columns are also summed together. The summed pixel outputs result in an image with fewer numbers of image pixels, but high signal-to-noise ratios.
In addition to providing for the proper timing and operation of the transistors in a pixel for purposes of acquiring and reading-out signals from the imager, the control signals generated by a row driver are also used to bias the transistor gates to protect the pixel from the effects of blooming. Blooming occurs when the amount of charge generated at a pixel exceeds the storage capacity of the pixel and the excess charge overflows into neighboring pixels. This may occur if the integration period is too long or the light incident on the pixel is too bright. One way to protect against blooming is to bias the transfer and reset transistors in a pixel so that the photosensor and floating diffusion region are drained of excess charge.
Unfortunately, the effectiveness of using this bias method to provide anti-blooming protection has been limited by the scaling and operating mode of the imager. For example, when an imager is operating in skipping or windowing mode, anti-bloom methods have not provided anti-bloom draining to skipped rows. Consequently, pixels in skipped rows have acquired charges that have leaked from other pixels, and no provision for the draining of these charges is provided since these pixels are not expected to integrate or acquire any charge. Should the photosensor or floating diffusion region of these skipped pixels overflow, neighboring integrating pixels are affected. Additionally, in four-way shared pixels, wherein pixels that are on different rows share a floating diffusion region, the accumulation of excess unwanted charge in a floating diffusion region of a pixel in a skipped row can contaminate the photo-generated charge accumulated by a selected row. Thus, there is a need to provide anti-blooming protection to skipped rows when an imager operates in skipping or windowing mode.
For similar reasons, there is a need to provide anti-blooming biasing to more than just a localized region of the pixel array when an imager is utilizing a rolling shutter. By applying only localized anti-blooming protection during a rolling shutter, neighboring, non-protected pixel rows can acquire excess unwanted charges and then contaminate other pixel rows.